The present invention relates to a magnetic recording medium used in an apparatus such as a magnetic disk apparatus; a process for producing the magnetic recording medium; and a magnetic recording and reproducing apparatus. More particularly, the present invention relates to a magnetic recording medium exhibiting excellent read-write conversion characteristics.
Recently, recording density of hard disk apparatuses, which are a type of magnetic recording and reproducing apparatus, has been increasing by 60% per year, and this trend is expected to continue in the future. Therefore, a magnetic recording head and a magnetic recording medium which are suitable for realization of high recording density have been developed.
A magnetic recording medium used in a hard disk apparatus or the like basically includes a structure as described below. On a substrate containing an Al alloy coated with Ni-P through plating or on a glass substrate, a non-magnetic undercoat layer for determining crystal orientation of a Co alloy layer is formed from Cr or a Cr alloy such as CrW or CrMo through sputtering among other methods. A thin film of Co alloy, serving as a magnetic layer, is formed on the non-magnetic undercoat layer. In addition, a protective film predominantly containing carbon is formed on the magnetic layer, and a lubricant such as perfluoropolyether is applied onto the protective film to form a lubrication film.
In accordance with an increase in recording density of an apparatus such as a magnetic disk apparatus, there has been demand for a magnetic recording medium exhibiting excellent read-write conversion characteristics. Such medium exhibits magnetic anisotropy in a circumferential direction. Therefore, a magnetic recording medium including an aluminum alloy substrate coated with an NiP film through plating (hereinafter the substrate may be referred to as an xe2x80x9caluminum substratexe2x80x9d) is provided with magnetic anisotropy in a circumferential direction by forming grooves mechanically on the NiP film in a circumferential direction (hereinafter the procedure will be referred to as xe2x80x9cmechanical texturingxe2x80x9d).
Non-magnetic substrates, for example, glass substrates have been used in magnetic recording media, because glass substrates exhibit rigidity, excellent impact resistance, and evenness. Thus, glass substrates are applicable to an increase in recording density of magnetic disk apparatuses, in which the flying height of a magnetic head is reduced. However, mechanical texturing cannot be satisfactorily carried out on a glass substrate, and thus glass substrates have been used mainly in magnetic recording media exhibiting magnetic isotropy. Even when glass substrates are subjected to texturing, satisfactory magnetic anisotropy is not obtained, and thus glass substrates have been used mainly in magnetic recording media exhibiting magnetic isotropy.
In order to solve such problems, studies have been performed on techniques for imparting magnetic anisotropy to a magnetic recording medium including a glass substrate. For example, Japanese Patent Application Laid-Open (kokai) Nos. 4-29561 and 9-167337 disclose formation of a hard film on a non-metallic substrate, which can be subjected to texturing. Japanese Patent Application Laid-Open (kokai) No. 5-197941 discloses a hard film formed through sputtering, and subjected to texturing. Japanese Patent Application Laid-Open (kokai) Nos. 4-29561 and 9-167337 disclose formation of a hard film on a non-metallic substrate, which can be subjected to texturing. However, in each of the magnetic recording media disclosed in these publications, a hard film is formed through electroless plating. Consequently, the production process for the medium includes complicated steps, resulting in high production costs. Japanese Patent Application Laid-Open (kokai) No. 5-197941 discloses an NiP hard film formed through sputtering. However, after the NiP hard film is formed through sputtering, the film must be subjected to mechanical texturing. Consequently, the production process for the magnetic recording medium disclosed in this publication includes complicated steps, resulting in high production costs. Therefore, there has been a strong demand for a production process for a magnetic anisotropic medium in which, even when a non-metallic substrate is employed, the medium is produced through a simple production process at low cost, similar to the case in which an aluminum substrate is employed.
In view of the foregoing, an object of the present invention is to provide an inexpensive magnetic recording medium exhibiting excellent read-write conversion characteristics, which includes a non-metallic substrate. The present inventors have performed extensive studies on the relation between the surface form of a magnetic recording medium and read-write characteristics of the medium suitable for realization of high recording density. The following embodiments of the present invention has been accomplished on the basis of these studies.
1) A first embodiment for solving the aforementioned problems provides a magnetic recording medium comprising a non-metallic substrate including, on its surface, texture grooves having a line density of 7,500 lines/mm or more; an orientation-determining film; a non-magnetic undercoat layer; and a magnetic layer, the film and the layers being formed on the substrate.
2) A second embodiment for solving the aforementioned problems is drawn to a specific embodiment of the magnetic recording medium according to 1), wherein the line density of the texture grooves is 15,000 lines/mm or more.
3) A third embodiment for solving the aforementioned problems is drawn to a specific embodiment of the magnetic recording medium according to 1), wherein the line density of the texture grooves is 20,000 lines/mm or more.
4) A fourth embodiment for solving the aforementioned problems is drawn to a specific embodiment of the magnetic recording medium according to any one of 1) through 3), wherein the Young""s modulus of the non-metallic substrate is 70-90 GPa.
5) A fifth embodiment for solving the aforementioned problems is drawn to a specific embodiment of the magnetic recording medium according to any one of 1) through 4), wherein the micro-waviness (Wa) of the surface of the non-metallic substrate is 0.3 nm or less.
6) A sixth embodiment for solving the aforementioned problems is drawn to a specific embodiment of the magnetic recording medium according to any one of 1) through 5), wherein the arithmetic average roughness (Ra) of at least one of a cutout portion and a side edge portion constituting a chamfer section at an end portion of the non-metallic substrate is 10 nm or less.
7) A seventh embodiment for solving the aforementioned problems is drawn to a specific embodiment of the magnetic recording medium according to any one of 1) through 6), wherein the non-metallic substrate is a glass substrate.
8) An eighth embodiment for solving the aforementioned problems is drawn to a specific embodiment of the magnetic recording medium according to 7), wherein the glass substrate comprises glass ceramic.
9) A ninth embodiment for solving the aforementioned problems is drawn to a specific embodiment of the magnetic recording medium according to 8), wherein the mean size of crystal grains contained in the glass ceramic is 10-100 nm.
10) A tenth embodiment for solving the aforementioned problems is drawn to a specific embodiment of the magnetic recording medium according to 8) or 9), wherein the density of crystal grains contained in the glass ceramic is 30-5,000 grains/xcexcm2 at the surface of the substrate.
11) An eleventh embodiment for solving the aforementioned problems is drawn to a specific embodiment of the magnetic recording medium according to any one of 1) through 10), wherein the orientation-determining film comprises any one selected from among a Cr alloy, NiB, NiP, and NIPZ (wherein Z is one or more elements selected from among Cr, Mo, Si, Mn, W, Nb, Ti, and Zr).
12) A twelfth embodiment for solving the aforementioned problems is drawn to a specific embodiment of the magnetic recording medium according to any one of 1) through 11), wherein the surface of the orientation-determining film has been exposed to an oxygen atmosphere.
13) A thirteenth embodiment for solving the aforementioned problems is drawn to a specific embodiment of the magnetic recording medium according to any one of 1) through 12), wherein the non-magnetic undercoat layer comprises Cr or a CrX alloy (wherein X is one or more elements selected from Mo, V, and W).
14) A fourteenth embodiment for solving the aforementioned problems is drawn to a specific embodiment of the magnetic recording medium according to any one of 1) through 13), wherein the predominant crystal orientation plane of Cr or a Cr alloy contained in the non-magnetic undercoat layer is a (200) plane.
15) A fifteenth embodiment for solving the aforementioned problems is drawn to a specific embodiment of the magnetic recording medium according to any one of 1) through 14), wherein the magnetic layer comprises a CoCrPtB- or CoCrPtBY-based alloy (wherein Y is one or more elements selected from among Ta and Cu).
16) A sixteenth embodiment for solving the aforementioned problems is drawn to a specific embodiment of the magnetic recording medium according to any one of 1) through 15), wherein a non-magnetic intermediate layer is provided between the non-magnetic undercoat layer and the magnetic layer.
17) A seventeenth embodiment for solving the aforementioned problems is drawn to a specific embodiment of the magnetic recording medium according to any one of 1) through 16), wherein the magnetic anisotropic index (ORxe2x95x90Hc in a circumferential direction/Hc in a radial direction) of the magnetic layer is 1.05 or more.
18) An eighteenth embodiment for solving the aforementioned problems provides a process for producing a magnetic recording medium, which process comprises a texturing step for forming texture grooves having a line density of 7,500 lines/mm or more on the surface of a non-metallic substrate; and a step for forming, on the non-metallic substrate, an orientation-determining film, a non-magnetic undercoat layer, and a magnetic layer.
19) A nineteenth embodiment for solving the aforementioned problems is drawn to a specific embodiment of the production process for a magnetic recording medium according to 18), wherein the line density of the texture grooves is 15,000 lines/mm or more.
20) A twentieth embodiment for solving the aforementioned problems is drawn to a specific embodiment of the production process for a magnetic recording medium according to 18) or 19), wherein the process further comprises a step for exposing the surface of the orientation-determining film to an oxygen atmosphere after the film is formed.
21) A twenty-first embodiment for solving the aforementioned problems is drawn to a specific embodiment of the production process for a magnetic recording medium according to 20), wherein the orientation-determining film is exposed to an oxygen atmosphere without being exposed to the outside air.
22) A twenty-second embodiment for solving the aforementioned problems is drawn to a specific embodiment of the production process for a magnetic recording medium according to 20) or 21), wherein the oxygen atmosphere is an atmosphere containing oxygen gas at a pressure of 5xc3x9710xe2x88x924 Pa or more.
23) A twenty-third embodiment for solving the aforementioned problems provides a magnetic recording and reproducing apparatus comprising a magnetic recording medium and a magnetic head for recording data onto the medium and reproducing the data therefrom, wherein the magnetic recording medium is a magnetic recording medium as recited in any one of 1) through 17).